Shielded electron microscope



March 11, 1941. E. RUSKA SHIELDED ELECTRON KICROSYCOPE Filed Feb. 4, 1939 INVENTOR.

ERNST BY ATTORNEYS Patented Mar. 11, 1941 UNITED STATES PATENT OFFICE.

SHIELDED ELECTRON BHCROSCOPE corporation of Germany Application February 4, 1939, Serial No. 254,534 In Germany February 1938 6 Claims.

This invention relates to an electron microscope having shielding means for reducing the disturbing influence of extraneous magnetic fields.

The resolving power of an electron microscope is considerably greater thanthat of the usual light microscope. The greater resolving power, however, can be satisfactorily utilized only if special precautions are taken to eliminate or reduce certain faults peculiar to the electron microscope. Besides the faults due to phenomena originating in the electron-optical system itself such as the diffraction of the electron Waves by the electron-optical objective, the influences of the space charge exerted on the rays, the dispersion of the electrons in the object, and the influence of the speed distribution of the electrons within the electron emitting source, there are faults to be put up with which are caused by eX- traneous disturbances, in particular by the influence on the electron rays of external magnetic fields. Such disturbing fields may be produced by machines or apparatus in the neighborhood of the electron microscope or by intensity fluctuations of the earthmagnetic field. Particularly on account of the long period of the fluctuation of the earths held a very heavy shielding of the electron microscope is necessary if the usual ferro-magnetic shielding means are employed. Such a shielding considerably increases the amount of material as well as the weight and the space requirements of the microscope. In certain cases, further, a particularly solid foundation for the electron microscope is necessary.

It is an object of the present invention to over come the just-mentioned disadvantages and, in particular, to reduce the requirements of the shielded electron microscope as to the amount, weight and compass of the shielding means.

According to the invention, the preferably ferro-magnetic shielding means of an electronmicroscope are substantially limited to approximately that portion of the electron-optical system which extends between the object and the projection coil, in particular between the electron-optical objective lens and the projection coil. According to another feature of the invention, the shielding means are composed of several parts in order to further reduce their weight. It has been found that such a partial shielding sufiiciently reduces the influence of extraneous magnet fields despite the diminution in space and weight obtained thereby.

In the accompanying drawing are shown two embodiments of the invention in diagrammatic form in sectional elevation.

Fig. 1 shows diagrammatically a magnetic electron microscope. l denotes the cathode, 2 the anode, 3 the condensing coil, 4 the objective coil and 5 the projection coil, The projection screen 6 is arranged on the bottom of the vacuum vessel enclosing the electron-optical elements of the microscope. The'window 8 serves to observe the projection screen. The shielding arrangement according to the invention consists of a plurality of concentrically arranged metallic cylinders 9 placed between thecoils 4 and 5. Such a shield ing arrangement is sufliciently effective and presents the further advantage that above the plane in which the object lies the electron ray may be moved in any direction over the object by mechanical or preferably, by electron-optical means;

In Fig. 2, the individual parts of the electron microscope are denoted by the same reference numerals as in Fig. l. The shielding arrangement consists, however, in this case of a cylinder l-2 of metal or ferromagnetic material, to the inner wall of which are secured a plurality of discs 9. The metal cylinder may also serve as the wall of the microscope. Magnetic disturbing fields penetrating from the outside are made ineffective by the cylinder of magnetic material and by the discs consisting of the same material. The shielding arrangement is closed in both embodiments at the ends but for an aperture for the passage of the electron rays. In both embodiments the coils are, as usual, enclosed in metal housings, the latter forming so to say an extension of the shielding arrangement shown in the drawing.

Electron microscopes according to the invention have a small weight, but, virtually, do not render the indispensable shielding arrangement less effective against disturbing external magnetic fields than in the case of a completely shielded electron microscope.

What is claimed is:

1. In an electron microscope having a vacuum vessel, means in said vessel for directing an electron beam on an object, and an electron-optical lens system comprising an objective lens and a projection lens for producing a magnified image of said object, in combination, a structure arranged in said vessel .and composed of several ferrometallic cylinders for protecting said beam from disturbances by extraneous magnetic fields, said cylinders being arranged one within the other concentrically to one another and to the optical axis of said system and extending between said objective lens and said projection lens.

2. In an electron microscope having means for directing an electron beam on an object, and an electron-optical lens system comprising an objective lens'and a projection lens for'producing a magnified image of said object, in combination, shielding means for preventing disturbances by extraneous magnetic fields from affecting said beam, said shielding means comprising a plurality of metal cylinders arrangedconcentrically to one another and to the axis of said system between said objective coil and saidprojection coil, the axial lengths of said cylinders being greater with increasin distance from said axis.

3. In an electron microscope having means for directing an electron beam on an object, and an electron-optical lens system comprising an objective lens and' a projection lens for producing a magnified image of said object, in combination, shielding means for preventing disturbancesby extraneous magnetic fields from afiecting said beam, said shielding means comprising a plurality of metal cylinders arranged concentrically to one another and to the axis of said system between said objective coil and said projection coil, and a metal plate covering the ends of said cylinders facing said. objective coil, said plate extending radially with respect to said optical axis and havinglacentral aperture of large diameter relative to that of said beam at said plate, said cylinders having difierent axial lengths, the axial lengths of said cylinders being greater with increasing distance from said axis.

4.7 In an electron microscope having a vacuum vessel, means in said vessel for directing an electron beam on an object, and an electron-optical lens system comprising an objective lens and a projection lens for producing a magnified image of said object, in combination, shielding means forfpreventing disturbances by extraneous magnetic metal extending in said vacuum vessel between said objective coil and said projection coil coaxially with said system, and several ferromagnetic plates mounted in said cylinder so as to extend radiall with respect to the optical axis of said system, said plates being spaced from one another along said axis and having each a central aperture.

5. In an electron microscope having means for directing an electron beam on an object, and an electron-optical lens system comprising an objective lens and a projection lens for producing a magnified image of said object, in combination, shielding means for preventing disturbances by extraneous magnetic fields from affecting said beam, said shielding means comprising a ferromagnetic cylinder extending between said objective coil and said projection coil coaxially with said system, and several ferromagnetic plates mounted in said cylinder soas to extend radially with respect to the optical axis of said system, said plates being spaced from one another along said axis and having each a central aperture, the diameter of said apertures being large relative to the local diameter of said beam and increasing towards said projection lens.

6. In an electron microscope having a vacuum vessel, means in said vessel forv directing an electron beam on an object, and an electron-optical lens system comprising an objective lens and a projection lens for producing a magnified image of said object, in combination, a structure of fer- I romagnetic sheet metal arranged in said vacuum vessel concentrically to the optical axis of said system for protecting said beam from disturbances by extraneous magnetic fields, said structure being separate from said lenses and extending substantially only over the portion of the electron-optical system between said object lens and said projection lens, said structure comprising a plurality of ferromagnetic elements arranged along said portion.

ERNST RUSKA. 

